Wireless Communications
Paging Systems
Cordless Telephone Systems
Cellular Telephone Systems (GSM, 3G)
Satellite Communication Systems
Globe Position Systems (GPS)
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Reference:
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Jagoda. A. Mobile Communications
Gaglkiardi R.M. Satellite Communications
http://www.mobileworld.org/gsm.html
http://www.iec.org/tutorials/cell_comm/#
http://www.itu.int/imt/
http://sss-mag.com/sat.html
Chapter 1 Cellular Telephone Systems
Cellular telephone system has developed from first-generation analog to
second-generation digital communications, and finally to the thirdgeneration IMT-2000.
1.1 Mobile Communications Principle
1.1.1 Early Mobile Telephone System Architecture
Each mobile uses a separate, temporary radio channel to talk to the cell site. The cell
site talks to many mobiles at once, using one channel per mobile. Channels use a pair
of frequencies for communication—one frequency (the forward link) for transmitting
from the cell site and one frequency (the reverse link) for the cell site to receive calls
from the users. Radio energy dissipates over distance, so mobiles must stay near the
base station to maintain communications. The basic structure of mobile networks
includes telephone systems and radio services. Where mobile radio service operates in
a closed network and has no access to the telephone system, mobile telephone service
allows interconnection to the telephone network (see Figure 1).
Traditional mobile service was structured in a fashion similar to television
broadcasting: One very powerful transmitter located at the highest spot in an
area would broadcast in a radius of up to 50 kilometers.
. Figure 2 shows a metropolitan area configured as a traditional mobile telephone
network with one high-power transmitter
1.1.2 Mobile Telephone System Using the Cellular Concept
The cellular concept structured the mobile telephone network in a different way.
Instead of using one powerful transmitter, many low-power transmitters were placed
throughout a coverage area.
For example, by dividing a metropolitan region into one hundred different areas (cells)
with low-power transmitters using 12 conversations (channels) each, the system
capacity theoretically could be increased from 12 conversations—or voice channels
using one powerful transmitter—to 1,200 conversations (channels) using one hundred
low-power transmitters.
The cellular radio equipment (base station) can communicate with mobiles as long as
they are within range. Radio energy dissipates over distance, so the mobiles must be
within the operating range of the base station. Like the early mobile radio system, the
base station communicates with mobiles via a channel. The channel is made of two
frequencies, one for transmitting to the base station and one to receive information
from the base station.
By reducing the radius of an area by 50 percent, service providers could increase the
number of potential customers in an area fourfold.
Systems based on areas with a one-kilometer radius would have one hundred times
more channels than systems with areas 10 kilometers in radius.
Unfortunately, Interference problems caused by mobile units using the same channel
in adjacent areas proved that all channels could not be reused in every cell. Areas had
to be skipped before the same channel could be reused. Even though this affected the
efficiency of the original concept, frequency reuse was still a viable solution to the
problems of mobile telephony systems.
Engineers discovered that the interference effects were not due to the distance
between areas, but to the ratio of the distance between areas to the transmitter power
(radius) of the areas. By reducing the radius of an area by 50 percent, service
providers could increase the number of potential customers in an area fourfold.
Systems based on areas with a one-kilometer radius would have one hundred times
more channels than systems with areas 10 kilometers in radius. Speculation led to the
conclusion that by reducing the radius of areas to a few hundred meters, millions of
calls could be served.
The cellular concept employs variable low-power levels, which allow cells to be sized
according to the subscriber density and demand of a given area. As the population
grows, cells can be added to accommodate that growth. Frequencies used in one cell
cluster can be reused in other cells. Conversations can be handed off from cell to cell
to maintain constant phone service as the user moves between cells
Advantages of cell structures:
higher capacity, higher number of users
less transmission power needed
more robust, decentralized
base station deals with interference, transmission area etc. locally
Problems:
fixed network needed for the base stations
handover (changing from one cell to another) necessary
interference with other cells
1.1.3 Cellular Access Technologies
Multiple access schemes are used to allow many mobile users to share simultaneously
a finite amount of radio spectrum. The sharing of spectrum is required to achieve high
capacity by simultaneously allocating the available bandwidth (or the available mount
of channels) to multiple users. For high quality communications, this must be done
without severe degradation in the performance of the system.
duplexing, is generally required in wireless telephone systems.
Duplexing may be done using frequency or time domain techniques.
Frequency division duplexing (FDD) provides two distinct bands of frequencies for every user.
The forward band provides traffic from the base station to the mobile, and the reverse band
provides traffic from the mobile to the base. In FDD, any duplex channel actually
consists of two simplex channel.
Time division duplexing (TDD) uses time instead of frequency to provide both a forward and
reverse link. If the time split between the forward and reverse time slot is small, then the
transmission and reception of data. appears simultaneous to the user.
TDD allows communication on a single channel (as opposed to requiring two simplex
or dedicated channels) and simplifies the subscriber equipment since a duplexer is not
required.
Multiple Access
There are three common technologies used by cell phone networks for
sharing the available bandwidth:
Frequency division multiple access (FDMA) assigns
individual channels to individual users.
It can be seen from Figure 8.2 that each user is allocated a unique frequency band or channel.
These channels are assigned on demand to users who request service. During the period of the
call, no other user can share the same frequency band. In FDD systems, the users are assigned a
channel as a pair of frequencies; one frequency is used for the forward channel, while the other
frequency is used for the reverse channel.
FDMA is used mainly for analog transmission
Time division multiple access (TDMA) systems divide the radio spectrum
into time slots, and in each slot only one user is allowed to either transmit or
receive.
It can be seen from Figure 8.3 that each user occupies a cyclically repeating time slot, so a channel may be thought of as particular time slot that reoccurs
every frame, where N time slots comprise a frame. TDMA systems transmit data
in a buffer-and-burst method, thus the transmission for any user is noncontinuous. This implies that, unlike in FDMA systems which accommodate analog FM,
diwtal data and diwtal modulation must be used with TDMA. The transmission
from various users is interlaced into a repeating frame structure as shown in
Figure 8.4. It can be seen that a frame consists of a number of slots. Each frame
is made up of a preamble, information message, and trail bits.
CDMA gives a unique code to each call and spreads it over the available
frequencies.
.All users in a CDMA system, as seen from Figure 8.5, use the same carrier frequency
and may transmit simultaneously. Each user has its own pseudorandom codeword
which is approximately orthogonal to all other codewords. The receiver performs a
time correlation operation to detect only the specific desired codeword. All other
codewords appear as noise due to decorrelation. For detection of the message signal,
the receiver needs to know the codeword used by the transmitter. Each user
operates independently with no knowledge of the other users.